Because of their excellent properties such as heat resistance (upper use temperature) and chemical resistance, tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymers are molded by such techniques as injection molding, blow molding, transfer molding, and melt compression molding, for use in pipes, joints, chemical storage vessels, in semiconductor manufacturing and at chemical plants, and used for lining pipes, tanks, and other containers.
The copolymer for such applications must be highly resistant to stress cracking. This type of mechanical durability can be improved by increasing in the perfluoro(alkyl vinyl ether) content of the copolymer, which however, results in reduced upper use temperature and increased manufacturing cost. Stress crack resistance can also be improved by increasing the copolymer molecular weight, but this results in reduced melt flow rate, which affects melt processibility adversely. It is an object of the invention to provide a copolymer composition which is excellent in mechanical durability and melt processibility utilizing only a small incorporated amount of a perfluoro(alkyl vinyl ether).